A boosted engine may exhibit higher combustion and exhaust temperatures than a naturally aspirated engine of similar output power. Such elevated temperatures may contribute to increased nitrogen-oxide (NOX) emissions and may accelerate materials ageing in the engine system, including exhaust-aftertreatment catalyst ageing. Exhaust-gas recirculation (EGR) is a popular strategy for combating these effects. EGR works by delivering exhaust gas having reduced oxygen content to the intake, which results in lower combustion and exhaust temperatures. In particular, EGR variants that deliver cooled EGR are desirable because they can supply a relatively large flow of exhaust gas to the intake. However, cooled EGR is liable to cause transient control difficulties in boosted engine systems, particularly in combination with spark-ignition. For instance, throttle closure in a system configured for cooled EGR may trap a significant volume of compressed, EGR-diluted air upstream of the throttle. Such trapping may occur on transitioning from high to low engine load, for example. Under low-load, closed-throttle conditions, however, the engine may require fresh air to sustain combustion. Opening a compressor by-pass valve at this time provides a partial, but incomplete remedy for the problem, as the EGR-diluted air remains upstream of the throttle, albeit at a lower absolute pressure.
Other approaches have targeted transient control issues in engine systems having cooled EGR. For example, U.S. Pat. No. 6,470,682 to Gray, Jr. provides a base intake manifold through which air and cooled LP EGR are provided to a diesel engine, and, an additional intake manifold that supplies only fresh air to the engine. The additional intake manifold is sourced by a fast-acting, electrically driven air compressor. When torque demand increases rapidly, the fast-acting compressor is switched on, displacing the existing mixture of air and EGR in the base intake manifold and providing increased oxygen mass to the engine, for increased torque. However, this system is particular to diesel engines, which may be unthrottled, and may tolerate significant amounts of EGR even at idle. Thus, the particular transient-control issues addressed in the reference differ from those experienced in spark-ignition engines.
The inventors herein have recognized that improved transient control in an EGR equipped engine system can be achieved by delivering boosted, EGR-diluted air and fresh air through separate throttle valves. In one embodiment, therefore, a method for providing intake air to an engine in a vehicle comprises forming a mixture of fresh air and treated exhaust, and compressing the mixture upstream of a first throttle valve coupled to an intake manifold of the engine. The method further comprises, during a higher engine-load condition, admitting the mixture to the intake manifold via the first throttle valve, and, during a lower engine-load condition, admitting fresh air to the intake manifold via a second throttle valve. In this manner, pressurized, EGR-diluted air remains trapped behind the first throttle valve, thereby alleviating at least some transient-control difficulties associated with cooled EGR.
Another method for providing intake air to an engine in a vehicle comprises delivering compressed fresh air and EGR to the engine via first and second throttle valves coupled to an intake manifold of the engine. During a higher engine-load condition, an EGR exhaust flow is cooled in a heat exchanger and the cooled EGR exhaust flow is admitting to the intake manifold. During a lower engine-load condition, fresh air is warmed in the heat exchanger, and the warmed fresh air is admitted to the intake manifold. In this manner, the heat exchanger serves double duty, abating knock at high engine load, and providing other advantages at low engine load. In particular, in the low-load region of the engine map the intake air can be heated to a significant degree without inducing knock; such heating reduces the density of the gas in the intake manifold with little or no reduction in inlet air pressure. Besides reducing pumping losses, increased intake air temperature may improve combustion reliability and increase EGR tolerance.